A rather lovely genital piercing between a man's scrotal sack and anus. IANAD but apparently it is situated over the prostate.
It takes a long time to heal (mine took months but I do heal slowly) but I'd say is worth it (if you're into that kind of thing). It makes riding a bike a lot more fun sometimes.
You can also enjoy the look of suprise on a lover's face when they see it for the first time.
Some folks hang weights from theirs but I've never tried it. I find it feels real good to rub the ring where it lies under the skin and can feel good when it's pulled gently (REALLY gently) but I've gone off that since last time somebody did it I felt like I was going to puke.